Traditionally, machining of metal workpieces to produce desired articles, such as hobbing processes to produce spur and helical gears, shafts, splines, and the like, has been performed in the presence of a coolant medium supplied to the point of engagement of a tool and workpiece. Aside from the obvious function of cooling the tool and workpiece, coolant also reduces tool wear and serves to flush metal chips, which emanate from the machined workpiece, away from the area of engagement of tool and workpiece and out of the machine. Once flushed away from the tool and workpiece, chips may separated from the coolant by filtering or by magnetic separator means such as, for example, those disclosed in U.S. Pat. Nos. 3,094,486 to Goeltz or 3,537,586 to Hunkeler.
While coolant certainly has many advantages, it also has its drawbacks. Coolant is expensive to purchase, and in some cases disposal costs are just as expensive. Coolant mist and coolant oil smoke are considered to be environmental hazards. Therefore, machines must include a mist/smoke collector as a means to remove such airborne contaminants from the atmosphere within the machine enclosure. Coolant circulation in a machine tool requires a pump and hoses to deliver coolant to the machining area, and a chip separator to remove metal chips from the coolant. Such separators are somewhat more complicated than simple powered drag lines used to convey dry chips. In some cases, filters may be needed to remove other debris from the coolant, or a coolant chiller may be required to control both the coolant and the machine equilibrium temperature.
Recently, dry machining processes such as dry hobbing have drawn attention as an alternative to processes utilizing coolant (wet machining processes) and discussed in Phillips, "New Innovations in Hobbing--Part II", Gear Technology, November/December 1994, pp. 26-30. However, in the absence of coolant, temperatures of both the tool and the machine are considerably higher. Hence, increased tool wear, due to thermal effects and the lack of an extreme pressure lubricant at the point of chip formation, becomes a serious issue. Materials such as high speed steel coated with titanium aluminum nitride, and tungsten carbide coated with titanium nitride, have demonstrated some success at withstanding dry hobbing process conditions. Laboratory success has also been demonstrated with coated ceramic-metallic (cermet) hobs, which cannot operate in contact with a coolant due to the susceptibility of these materials to cracking from thermal shock.
It has been stated by some that carbide hobbing can be performed at a faster rate in a dry process than when coolant is applied. However, current production evidence suggests that this may only be the case when tool coatings improve beyond their current level, since an acceptable tool life becomes a severe problem as dry hobbing rates are increased. Nonetheless, such improvements should be expected over time.
It may be seen that dry hobbing has the potential to overcome many serious and costly drawbacks associated with the use of a liquid coolant. Also, dry chips are normally more valuable as a recyclable material than chips which are residually wetted by a process fluid. Parts cut without coolant do not need washing, prior to further processing such as heat treatment.
However, as discussed above, the heat generated in dry hobbing is a major contributor to tool wear but it also has detrimental effects on the machine itself, causing differential growth of components such as spindles, bearings, or the machine frame. Much of the process heat in dry machining is removed by the chips which must be removed from the machine as quickly as possible and in a manner by which they do not contact the machine frame for any extended period of time. One way to remove dry chips is to permit the hot chips to slide by gravity toward a chip conveyor built into the base of a hobbing machine. Such a chip removal system is shown in Ophey, "Gear Hobbing Without Coolant", Gear Technology, November/December 1994, pp. 20-24.
The workpiece itself is also relied upon as a means of removing heat from the cutting chamber in dry hobbing. Hot exiting workpieces must be tolerated, since they remove heat that would otherwise produce higher equilibrium temperatures in machine spindles, bearing housings, structures, and tooling. Certain kinds of common, effective workholding fixtures cannot be used during aggressive dry machining processes because of thermal growth or closure of fixture elements which would produce jamming rather than the free introduction and release of successive workpieces.
It can be seen that both wet and dry hobbing have their advantages as well as disadvantages and usage of either process is dependent upon the circumstances of the particular job. However, until now, it has not been possible to easily convert from wet-to-dry or dry-to-wet processes on a single machine without associated time consuming efforts such as draining (or filling) of coolant reservoirs, replacing chip hoppers, and removing (or installing) any coolant/chip separating equipment.
It is an object of the present invention to provide a machine tool capable of wet and dry machining with very little changeover time required to switch from wet-to-dry and dry-to-wet processes.
It is a further object of the present invention to provide a system to remove metal chips from a machine tool, regardless of wet or dry operations, in which the chips are transferred to separate outlets for wet and dry metal chips.
It is a further object of the present invention to provide a gear hobbing machine capable of wet and dry machining and having independent chip disposal outlets for wet and dry metal chips.
An additional object of the present invention is to provide a gear hobbing machine, employing the chip removal system referred to above, in which the upper deck surfaces of the cutting chamber and the workholding equipment can optionally be irrigated during the performance of a dry machining process to achieve improved machine thermal control. The falling chips would, in this arrangement, transfer their heat directly to the irrigation/coolant liquid, and in turn to all extremes of the machine base by circulation of the coolant after chip separation.
Also, the work fixture itself may be cooled by a liquid circulation sleeve with an outlet to the optional deck irrigation. Neither the optional deck irrigation nor the optional work fixture cooling means results in the wetting of the tool or the workpiece.